You always need a current limiting device when using an LED and a resistor is the simplest one you can have. Anyone who tells you different is wrong.

I was going to say the same thing but then I figured that someone would point out the exception, which would be if one happened to be using a current limiting supply. Even then the resistor wouldn't hurt anything.

But I'm confused: do I really need a resistor if I limit the PWM output to 3.3v (as in my code) ?

PWM 3.3v only means that the arduino is outputing 5v 3.3v/5v = 66% of the time, and 0v 33% of the time.

However, the answer to your question is yes and no.

Yes, you need a resistor for a proper design.

No, you don't need a resistor as the pin's internal limitations in its current capabilities serve as a "resistor", and the diodes are one of those electronic devices that can take a lot of abuses temporarily.

I would put one in myself. But I would encourage you to experiment by not putting one in just to see for yourself.

In case you are asking, I have shorted avr's output pins (when outputting a logic 1) without damaging any.

So I think I don't even need to add a resistor!Sorry, I'm still newb to electronics

Why do they always fight "the resistor"?Why resist?Is it "the expense"?

"You gotta fight - for your right - to party!"Don't react - Read."Who is like unto the beast? who is able to make war with him?"It's "bipolar transistor" or "junction transistor" - "BJT" is just stupid.When all else fails, check your wiring!

This on-off pattern can simulate voltages in between full on (5 Volts) and off (0 Volts) [...] The result is as if the signal is a steady voltage between 0 and 5v controlling the brightness of the LED.

So I supposed, limiting the PWM as in my code, will output a steady 3.3V. Then, I used a led resistor calculator (the parallel calculator, since I suppose the LEDs of my display are mounted parallel)... which tell me that I need a resistor of 1 Ohm (AKA: I don't even need a resistor), when the input voltage is the same as the forward voltage of the LED, and doesn't matter how much current I enter as the "desired LED current".

Guys, I'm lost...

Quote

Why do they always fight "the resistor"?Why resist?Is it "the expense"?

I don't, in fact I planned to add a resistor since the first post. I'm just wondering why I need one when a calculator tell me otherwise.

In addition to everything in the previous post you have to also understand that:

(1) You do not know what the forward voltage of the LED might be.(2) Even if you determine what it is for one diode under one particular set of circumstances it would be different ... (a) for a different diode under the same circumstances. (b) for the same diode under different circumstances. (c) for a different diode under different circumstances.

In other words - if you to try to design a circuit to drive an LED with some specific voltage you are doomed to failure.

Did you ever consider why it takes so long to earn an Engineering degree?

The answer will depend on your multi-meter. If it is a multimeter that reads "average" (an analog meter or an rms meter (not quite but close)), your math then works.

I think you are overthinking this. Put a resistor (start with something safe like 330) in, power it to rail (5v), and find a resistor value that gives you more light than you want, as your pwm will dial back the (average) current later.